1. Field of the Invention
This invention relates to an improvement in a limiter circuit, which, when the input applied to a low frequency amplifier is extremely high in level, automatically amplitude controls the output of the low frequency amplifier according to the level thereof, because in such a case the input is clipped and therefore the output is distorted.
2. Description of the Prior Art
One example, of a conventional limiter circuit of this type is shown in FIG. 1. The conventional limiter circuit will be described with reference to that figure.
In FIG. 1, reference numeral 1 designates a voltage gain controlled amplifier to which the output of a low frequency amplifier (not shown) is applied; 2, an amplifier for amplifying the output of the voltage gain controlled amplifier; 3, a time constant circuit for blocking or delaying the output of the amplifier 2 for a predetermined delay time; and 4, a rectifier circuit for converting the output of the time constant circuit 3 into a dc output.
In the limiter circuit thus organized, the output of the low frequency amplifier is applied to the voltage gain controlled amplifier 1. A part of the output of the amplifier 1 is amplified by the amplifier 2 and delayed for the predetermined period of time by the time constant circuit 3, and it is converted into a dc output by the rectifier circuit 4, and fed back to the voltage gain controlled amplifier 1. The application of the output to the rectifier circuit 4 is suspended by the time constant circuit 3 for the predetermined period of time. The aforementioned control voltage output from the rectifier circuit 4 changes with the output level of the low frequency amplifier, and therefore the output level is controlled in amplitude by the voltage gain controlled amplifier 1. Accordingly, as the output level of the low frequency amplifier increases, the output level is effectively controlled in amplitude by the voltage gain controlled amplifier 1 over the entire frequency band.
In the above-described conventional limiter circuit, the output level is damped over the entire frequency band. Therefore, the operation of the limiter circuit will cause a so-called "breathing effect" such that the level variation in the higher frequency range offends the ear.
When an excessively great input is applied, the low frequency band is especially considered because of the following reason: In general, operation of the limiter circuit is required when high frequency signals are superposed on low frequency signals. Since, with acoustic power taken as a reference, high frequency signals are originally lower in amplitude level than low frequency signals, high frequency signals are more difficult to clip than low frequency signals. Therefore, with high frequency signals, clipping distortion scarcely takes place, and such distortion is not so offensive to the ear as that with low frequency signals. Hence, it is unnecessary to effect the limiting operation over the entire frequency band; that is, the limiting operation should be effected only for the low frequency components.
In the case of a mobile audio device mounted on a vehicle, the electric power source is of the order of 12V, and therefore it is rather difficult to provide a high power amplifier; however, because of high road noise, it is positively necessary to increase the acoustic output, especially the lower frequency level acoustic output. In this case, with the amplifier operated using the electric power thus limited, the low frequency band is clipped.
Let us consider the case where limiter control is carried out for each channel. If, in this case, the channels differ from each other in the attenuation of high frequency components, then since the high frequency components are significant in directivity the sound image is variable in localization, which is offensive to the ear.
Furthermore, the voltage gain controlled amplifier 1 in the limiter circuit produces noise in the high frequency region, and therefore the output may include noise components attributable to the voltage gain controlled amplifier 1.